Pressure swing adsorption (PSA) processes provide a commercially attractive approach for separating and purifying at least one component of a feed gas mixture which contains at least one less readily adsorbable component and at least one more readily adsorbable component. In the case of air, the more readily adsorbable component is typically nitrogen, and the less readily adsorbable component is oxygen. Adsorption occurs in an adsorbent bed at an upper adsorption pressure with the more readily adsorbable component, e.g. the nitrogen component, thereafter being desorbed from the adsorbent bed by reducing adsorbent bed pressure to a low desorption pressure.
Multi-bed PSA processes are particularly effective for oxygen plant capacities in the size range of 30,000 NCFH to 120,000 NCFH or more of oxygen. For applications with flow requirements that fall below this range, a single-bed pressure swing adsorption system is desirable. In U.S. Pat. No. 5,370,728 to LaSala et al., entitled "Single Bed Pressure Swing Adsorption System and Process", assigned to the same Assignee as this application, a single bed PSA or VPSA (vacuum pressure swing adsorption) process is disclosed which utilizes a pair of external surge tanks, one such tank supplying high purity oxygen both as the desired product and also as a purge gas to the adsorbent bed during an evacuation step of the processing cycle. The second surge tank collects void space gas (i.e., low purity oxygen) which is withdrawn from the bed during depressurization and supplies that void space gas to the adsorption bed during a repressurization of the adsorbent bed.
In FIG. 1, a diagram is shown of the LaSala et al. system which incorporates a single adsorbent bed for oxygen production. A product surge tank, hereafter called "high purity oxygen tank", is employed together with an equalization tank (hereafter called "low purity oxygen tank") to enable product recovery and to improve power requirements of the system. Line 1 is used to supply feed air to a feed/vacuum blower 2 via a dust filter 13 gas silencer unit 3 and valve 4. Line 5 from feed/vacuum blower 2 connects to lines 6 and 7, with line 6 including valve 8 and outlet snubber unit 9 from which gas is discharged through line 10. Venting of the gas stream can also be accomplished by means of valve 15 through line 14, to which is connected to unit 9. Line 7 includes outlet snubber unit 11, aftercooler 12 for feed gas cooling and a valve 13. Discharge line 14 contains a valve 15. Line 16, containing valve 17, connects to line 1 downstream of valve 4. Lines 7 and 16 both connect to line 18 which extends from the bottom portion of an adsorbent bed within adsorption vessel 19. From the top of adsorption vessel 19, a line 20 extends to and connects with line 21, valve 22 and low purity oxygen tank 23. Line 24 connects with line 20 and, via a check valve 25, connects to high purity oxygen tank 26. As described below, check valve 25 is not required in the constant product make step embodiment of the subject invention. Instead, product gas is passed through valve 29 to high purity tank 26. Product oxygen is withdrawn from high purity oxygen tank 26 through line 27. Line 20 also connects, via a valve 29, to line 28 and high purity oxygen tank 26.
The operation of the system of FIG. 1, as described in the LaSala et al. patent, involves a five step cycle having the following sequence: (1) partial depressurization; (2) evacuation; (3) purge; (4) partial repressurization and (5) pressurization and product recovery. Assuming that high purity oxygen tank 26 has received its charge of high purity oxygen from adsorbent vessel 19 and that adsorbent vessel 19 is at an upper adsorption pressure, the processing sequence begins to recycle by partially depressurizing adsorbent vessel 19. Thus, valve 13 closes and valve 15 opens, enabling feed/vacuum blower 2 to vent air to the atmosphere. Valve 22 opens and adsorbent vessel 19 begins to depressurize from the upper adsorption pressure. Void gas is displaced from the void volume in the adsorbent bed and is passed through line 21 to low purity oxygen tank 23, building to a pressure of about 14.5-15 psia. The concentration of oxygen in low purity oxygen tank 23 is typically 85-89%. Valves 8, 17 and 29. are closed during this action, which continues until the pressure in adsorbent vessel 19 falls to an intermediate pressure, e.g., 16 psia. The approximate cycle time for this partial depressurization step is about 4-7 seconds.
The vessel evacuation step occurs after adsorbent vessel 19 has expelled a portion of void gas into low purity oxygen tank 23 and the pressure in adsorbent vessel 19 has dropped to the intermediate pressure. Valves 8 and 17 are opened and valves 4, 15, 13, 22 and 29 and check valve 25 are closed. Thus, gas in adsorbent vessel 19 is diverted out line 18, through valve 17, line 16, to the inlet of feed/vacuum blower 2. This void gas is discharged through outlet silencer 9 to the atmosphere. This action enables feed/vacuum blower 2 to further evacuate adsorbent bed vessel 19 to below atmospheric pressure.
The approximate composition of the evacuation gas, averaged over the evacuation portion of the cycle, is 90% nitrogen and 10% oxygen. Adsorbent vessel 19 is evacuated to below atmospheric pressure to cause the difference in partial pressures of the nitrogen gas in the void spaces of the adsorbent to desorb and thus regenerate the adsorbent to prepare for a next cycle. The vessel evacuation step takes place until the pressure in adsorption vessel 19 reaches a lower desorption pressure, e.g., approximately 5 psia. The step time for this cycle is about 25-40 seconds.
Next, a vessel purge step occurs at the lower desorption pressure. Valve 29 opens and a small side stream of product gas from high purity oxygen tank 26 is diverted into the top of adsorbent vessel 19. The oxygen input sweeps away a large portion of the remaining void gas in vessel 19 which is comprised mainly of desorbed nitrogen. The purge gas stream displaces the desorbed gas present in the void volume of the adsorbent vessel 19. The vessel purge step occurs at a constant vacuum or other desorption pressure level, with valves 8 and 17 remaining open, control valve 29 open and all other valves closed.
When most of the desorbed gas in the void spaces of the adsorbent in adsorption vessel 19 is replaced with the product gas (oxygen), the processing sequence advances to a partial repressurization step. The average length of time for the vessel purge step is approximately 7-10 seconds.
During the partial repressurization step, valves 8, 17 and 29 are closed and valves 4 and 15 are opened to allow feed/vacuum blower 2 to run unloaded. Control valve 22 is opened and void gas from low purity oxygen tank 23 (that was collected during the partial depressurization step) is used to repressurize adsorbent vessel 19 to an intermediate pressure level, e.g., 10 psia. The time for this step is approximately 4-7 seconds.
Now that the adsorbent bed in adsorbent vessel 19 has been partially repressurized to an intermediate pressure of about 10 psia, feed air is supplied from feed vacuum blower 2 during a pressurization/product recovery step of the cycle. Under these conditions, valves 4 and 13 are open and valves 8, 15, 17, 22 and 29 are closed. Check valve 25 is adjusted so that it opens when the pressure in adsorbent vessel 19 becomes greater than the pressure in high purity oxygen tank 26.
As feed air is introduced into adsorbent bed vessel 19, the pressure therein increases until it is equal to that in high purity oxygen tank 26. Check valve 25 then opens and product gas (i.e., oxygen) is fed to high purity oxygen tank 26. The supply of product gas continues until the pressure at the top of adsorbent vessel 19 reaches an upper adsorption pressure, typically about 22.5 psia. Now, high purity oxygen tank 26 is available to provide oxygen for downstream use, independent of adsorbent vessel 19. A typical time for this portion of the process is about 18-25 seconds.
Feed/vacuum blower 2 has a limited differential pressure capability, and exhibits lower efficiency at high compression ratios. Thus, it is desirable that the cycle minimize the operating vacuum level to reduce that pressure differential. Such action results in feed/vacuum blower 2 operating in a more efficient range and also results in elevated suction pressure, hence, increasing the waste capacity of the machine at higher efficiency. Further, both high separation efficiency and high adsorbent utilization are desirable to assure lowest power consumption and largest capacity for a given investment.
Accordingly, it is an object of this invention to provide an improved method of operation for a single bed, pressure swing adsorption system.
It is another object of this invention to provide a method for reducing differential pressure across a compressor utilized in a single bed pressure swing adsorption system.
It is yet another object of this invention to provide an improved single bed, pressure swing adsorption system wherein the time required for individual portions of the operating cycle are reduced, thereby enabling higher system efficiency.